There is evidence that malignant transformation is related to cell proliferation and is specifically dependent on DNA replication. Thus, the structural damage inflicted upon the genetic material by chemical carcinogens is necessary but not sufficient to initiate the process of chemical carcinogenesis. Only with the subsequent occurrence of DNA replication, those alterations not yet eliminated by error-free repair could be converted into actual transforming lesions. The probability that a cell will be transformed by a chemical carcinogen is related not only to the frequency of DNA damage (modulated by carcinogen dose and DNA repair) in specific DNA sequences (proto-oncogenes?), but also, to the molecular mechanisms of replication of structurally altered DNA. One of our main premises is that the process of neoplastic transformation is genetically determined and shares important mechanistic characteristics with the process of gene mutation. This could explain the S phase dependence of transformation and mutation observed in many systems. The higher susceptibility to malignant transformation observed when cells are treated in late GI or early S phase, as compared to other points in the cell cycle, is probably due to damage to specific cellular genes at or close to the time of their replication. Alternatively, a lower rate of repair during the S phase would also increase the probability of fixation of a genetic alteration, independently of the timing of replication of the damaged DNA sequence. In this Program Project we will pool our individual expertise to obtain solid experimental evidence to support or refute these hypotheses. We propose this Program Project Grant to bring together the goals and efforts described above for the individual grants into three major research areas: 1) phenomenological characterization of malignant transformation in three distinct model systems, i.e., C3H 10T1/2 and human fibroblasts in vitro and liver cells in vivo; 2) mechanistic studies of DNA replication and repair; 3) characterization of genetic and molecular mechanisms for transformation and mutations. In each of these areas the main emphasis will be on the cell-cycle variations of the different end-points mentioned above.